1. Field of the Invention
The present invention relates to a protective system in a brushless DC motor control circuit and more particularly to such protective circuits in electromobiles (electric cars).
2. Description of the Background of the Invention
In a DC motor found in an electromobile of the prior art, current is fed to coils on the rotor through sliding brushes and commutators. It is desirable to eliminate the contact wear and noise associated with these parts.
As a solution to the foregoing problems, electromobiles have been developed in which the aforementioned motor is replaced by a brushless DC motor composed of a rotor having permanent magnets and a stator coil (hereinafter "brushless DC motor"). To control such a brushless DC motor, three-phase sinusoidal waves are generated in a manner corresponding to the magnetic position of the rotor and are multiplied by a current command and fed to a pulse width converting circuit (invertor), in which they are converted into a motor current such as a sinusoidal phase current. This phase current is fed to the stator coil of the brushless DC motor. Moreover, the current is fed back and is controlled by an invertor circuit to match the current command.
However, the brushless DC motor of the prior art requires the detection of the magnetic position of the rotor by means of a magnetic position detector. If the magnetic position detector malfunctions or the motor control circuit is disconnected from the invertor circuit, the phase current cannot be generated in a manner for driving the brushless DC motor in a stable state (e.g., stopping the motor).